With the increase in civil use of what was considered during the “cold war” years “strategic” or “restricted” metals, such as zirconium and titanium, and the accompanying drop in their prices, an increasing number of consumer goods, medical, dental and orthopedic, civil engineering and architectural structural and decorative components, and other industrial as well as civil and military uses have been made of metals such as zirconium, titanium, and alloys thereof. With this increase in use, there has been a growing interest in their unique metallurgical properties and advantages as employed in known and new applications. These properties include very high tensile and yield strength, light weight, and chemical inertness together with its corollary hypoallergenic property, which makes these metals and alloys suitable for dental, orthopedic and other prostheses such as joint replacements, arterial stents, and cardiac valves, as well as for consumer fashion accessories that benefit from the same properties, such as body-piercings, wrist watches, sunglass frames, and the like.
Increased interest in these metals and their uses has been accompanied by demand for methods for providing hardened surfaces, for providing surfaces exhibiting reduced friction, and for improving surface appearance. At the same time, their strength, low elasticity and ductility has rendered them materials of choice for stealth activities, from hunting to law enforcement and the military, for which dark colors are preferred.
Anodizing is known for altering the color and surface appearance of titanium and niobium. Anodizing of these metals and certain of their alloys generates a thin, colorful outer layer on the metal, which wears off readily and is easily scratched, chipped, or otherwise removed.
U.S. Pat. No. 6,093,259 to Watanabe et al. teaches methods for providing various colored surfaces on titanium by treatment with aqueous alkaline solutions of KOH, NaOH and LiOH, applied singly or as a mixture, optionally accompanied by thermal treatment at moderate temperatures, and optionally comprising a nitriding process.
U.S. Pat. No. 5,037,438 to Davidson, and U.S. Pat. No. 5,169,597 to Davidson et al., disclose surface treatment of another cold war metal, zirconium, by thermal or salt bath oxidation within temperature ranges readily achievable by conventional kilns, for improving mechanical and metallurgical properties. The resulting smooth and very hard blackened surface reportedly reduced friction, increased scratch resistance, enhanced the strength of the metal immediately beneath the surface coating, and provided a blue/blackness colored surface. These enhancements were attributed to oxygen diffusion into the substrate metal, which also improved the fatigue properties of the metal.
In attempting to produce articles that require or would benefit from the combination of high tensile strength, hardness, scratch and wear resistance, and color control from dark gray to blackness, light weight, and hypoallergenicity, it is known that zirconium and titanium provide these benefits to varying degrees.
However, unalloyed titanium colored according to the method taught by Watanabe et al. does not exhibit enhanced resistance to wear and generally retains the properties of untreated titanium. Also, the method requires the use of hazardous materials, personal safety equipment such as gas masks, impermeable gloves, complete skin coverage, and the like.
Using unalloyed zirconium to the extent taught by Davidson, is limited to unalloyed zirconium or alloys containing at least 80% zirconium, and preferably from about 95% to about 100%, by weight. In contrast, Davidson et al. teach the use of a ternary alloy including niobium, adding cost and complexity compared to binary alloys. Davidson and Davidson et al. are primarily directed to weight bearing prosthetic implants, for which color control is relatively unimportant.
While unalloyed zirconium displays high tensile strength, hypoallergenicity, and a beneficial surface coating when oxidized, it is known that alloys containing both zirconium and titanium offer superior metallurgical properties compared to each metal alone. Yoshiaki, I. et al. “Improved Biocompatibility of Titanium-Zirconium (Ti—Zr) Alloy: Tissue Reaction and Sensitization to Ti—Zr Alloy Compared with Pure Ti and Zr in Rat Implantation” Mater. Trans. 46(10): 2260-2267 (2005) (teaching superior biocompatibility of Ti—Zr alloys compared to each metal alone).
Certain ratio ranges of zirconium to titanium exhibit superior mechanical properties compared to the component metals in the unalloyed state. Kobayashi, E. “Mechanical properties of the binary titanium-zirconium alloys and their properties for biomedical purposes” J. Biomed Materials Research 29(8) (1995). Alloys in the range of 1:1 zirconium:titanium by weight, disclosed for use as dental implants, exhibit hardness and tensile strength about 2.5 times as high as the unalloyed components. These results were reported for both cast and homogenized specimens.
Ternary alloys containing zirconium, titanium and a third metal are also known for applications including prostheses. U.S. Pat. No. 5,820,707 and to Amick et al. teach ternary alloys including a third metal selected from niobium, tantalum and vanadium. The third metal is taught as passivating the tendency of the zirconium and titanium to ignite and combust. Amick et al. teaches very high temperatures and long duration for complete or near complete oxidation of the alloy workpiece, which therefore requires passivation through the inclusion of the third metal in the alloy. The method reportedly provides smooth and hard surfaces, which for some alloys are described as being “blue/blackness”.
U.S. Pat. No. 6,759,134 to Rosenberg discloses ternary alloys containing titanium, niobium, and a third metal from the group consisting of zirconium, tantalum, molybdenum, hafnium, zirconium, chromium, with emphasis on alloys containing from 3% to 17% by weight niobium for its passivating properties and for the creation of a smooth and hard surface layer of niobium containing oxide with an aesthetic chromatic value.
However, Amick et al. and Rosenberg require at least a ternary alloy, do not teach control of the surface shade on a scale from dark gray to blackness, and do not teach the benefits of enhanced tensile strength of the treated alloy.
In sum, Yoshiaki et al. and Kobayashi et al. teach binary zirconium titanium alloys of specified weight ratio that possess good metallurgical, mechanical and hypoallergenic properties. The ternary alloys of Amick et al. and Rosenberg are more intricate and costly to produce and have not been shown to possess the additional strength and hypoallergenic benefits of the binary alloy. Davidson and Davidson et al. teach the benefits of zirconium based alloys comprising a zirconium oxide coating, while Rosenberg and Amick et al. offer combinations that rely upon the presence of niobium oxide in the coating, which form of the oxide was not shown to possess the same enhanced strength and fatigue resistance as the primarily zirconium oxide coating disclosed by Davidson.
While the prior art provides a subset of the group of properties required by and benefiting various articles, namely, high tensile strength, high hardness, low ductility and elasticity, enhanced fatigue resistance, and biocompatibility, it does not teach the capability to combine the full scope of all of these benefits, the advantages in the capability to have controllable shades of dark gray to black and nor does it offer the benefits of simplicity and cost reduction to be gained through the use of a binary alloy.
Therefore, there is a need in the art for alloys and surface coatings capable of providing articles exhibiting all of the potential beneficial properties available from zirconium titanium binary alloys. All this and more will become apparent to one of ordinary skill upon reading the following disclosure and claims.